Tracing the Cycling and Fate of the Explosive 2,4,6-Trinitrotoluene in Coastal Marine Systems with a Stable Isotopic Tracer, ¹⁵N-[TNT]

Abstract

2,4,6-Trinitrotoluene (TNT) has been used as a military explosive for over a hundred years. Contamination concerns have arisen as a result of manufacturing and use on a large scale; however, despite decades of work addressing TNT contamination in the environment, its fate in marine ecosystems is not fully resolved. Here we examine the cycling and fate of TNT in the coastal marine systems by spiking a marine mesocosm containing seawater, sediments, and macrobiota with isotopically labeled TNT ((15)N-[TNT]), simultaneously monitoring removal, transformation, mineralization, sorption, and biological uptake over a period of 16 days. TNT degradation was rapid, and we observed accumulation of reduced transformation products dissolved in the water column and in pore waters, sorbed to sediments and suspended particulate matter (SPM), and in the tissues of macrobiota. Bulk δ(15)N analysis of sediments, SPM, and tissues revealed large quantities of (15)N beyond that accounted for in identifiable derivatives. TNT-derived N was also found in the dissolved inorganic N (DIN) pool. Using multivariate statistical analysis and a (15)N mass balance approach, we identify the major transformation pathways of TNT, including the deamination of reduced TNT derivatives, potentially promoted by sorption to SPM and oxic surface sediments.

Review and synthesis of evidence regarding environmental risks posed by munitions constituents (MC) in aquatic systems

“…Mass balance calculations using 15 N nitro-labeled MC has been completed in complex mesocosms (Smith et al 2015b;Ballentine et al 2016). The system consisted of two experimental aquaria connected to a common reservoir.…”

“…The two experimental aquaria had 7 cm of sediment with Long Island Sound (LIS) seawater. Smith et al (2015b) experimentally traced the fate of TNT during three distinct phases. The first phase consisted of a single TNT pulse followed by five days of monitoring with the purpose of simulating a pulsed delivery of TNT to an uncontaminated system and simplifying the estimation of first order decay coefficient.…”

“…The final phase terminated the dosing and monitored the loss of TNT while evaluating the fate of TNT after additions had ceased. Smith et al (2015b) spiked the system with an initial water column concentration of 0.90 mg/L. TNT loss in the water column occurred quickly, within 24 hours of the initial spike, only 3% of the TNT remained.…”

“…TNT loss in the water column occurred quickly, within 24 hours of the initial spike, only 3% of the TNT remained. TNT was measured in the following pools from largest to smallest amounts: suspended particulate matter > phototrophic tissue > sediments (0-2 cm) > sediments (2-4 cm) > pore water > heterotrophic tissue (Smith et al 2015b). 15 N enrichment was observed in all dissolved inorganic nitrogen (DIN) pools with NH4 + being the largest DIN endpoint and only accounted for as much as 7% of the TNT derived N added to the system for Smith et al (2015b).…”

“…TNT was measured in the following pools from largest to smallest amounts: suspended particulate matter > phototrophic tissue > sediments (0-2 cm) > sediments (2-4 cm) > pore water > heterotrophic tissue (Smith et al 2015b). 15 N enrichment was observed in all dissolved inorganic nitrogen (DIN) pools with NH4 + being the largest DIN endpoint and only accounted for as much as 7% of the TNT derived N added to the system for Smith et al (2015b). However, the TNT in all of these pools only represented 27% of the total TNT introduced into the system, meaning that the majority of the 15 N was unaccounted for in the analytes measured (Smith et al 2015b).…”